A liquid crystal display element utilizes an optical anisotropy and a dielectric anisotropy which a liquid crystalline material possesses. As a display mode therefor, there are known a twisted nematic mode (TN), a super twisted nematic mode (STN), a dynamic scattering mode (DS), a guest-host mode (G-H), DAP mode and others. As a driving mode therefor, there are known a static-driving mode, a time-sharing driving mode, an active-matrix driving mode, a dual frequency driving mode and others. The properties of liquid crystalline materials used for these various liquid crystal display elements vary depending on the application of the elements, but liquid crystalline materials are required to be stable to external environmental factors such as moisture, air, heat, light, etc. and to show a liquid crystal phase over a wide temperature range around room temperature, with a lower viscosity and a lower driving voltage. In addition, a liquid crystalline material generally used for a liquid crystal display element is composed of several to twenty liquid crystalline compounds for providing the optimum dielectric anisotropy (.DELTA..epsilon.) or optical anisotropy (.DELTA.n) which is required for individual display elements. In view of this, there has been required a compatibility with other liquid crystalline compounds, particularly in recent years, a good low-temperature compatibility from the demand for application under various environments.
A liquid crystalline compound having as a substituent a fluorine atom at the end generally shows a lower dielectric anisotropy (.DELTA..epsilon.) and optical anisotropy (.DELTA.n) as compared with a compound having a cyano group as a substituent, but has a remarkably superior chemical stability to that of the cyano-substituted compound and is considered to cause a less production of ionic impurities due to a change with time. Therefore, the fluorine-containing compounds have been actively used for various modes including an active-matrix mode. The recent trend of development in this field is directed to making a small-sized liquid crystal element including a portable TV and lowering in driving voltage in compliance with the demand for a lower voltage. In order to achieve this object, the development of a compound having a high dielectric anisotropy (.DELTA..epsilon.) is active.
In order to increase a dielectric anisotropy (.DELTA..epsilon.) in the fluorine-containing compound, it is effective to increase the substitution number of a fluorine atom, which is a procedure usually carried out by those skilled in the art. However, it has been empirically realized by those skilled in the art that there is a proportional relationship between the substitution number of a fluorine atom and the viscosity of the compound, and further that there is an inverse relationship between the substitution number of a fluorine atom and the temperature range of a liquid crystal phase. Accordingly, it has been considered to be difficult to improve a dielectric anisotropy (.DELTA..epsilon.) only, while inhibiting an increase in the viscosity and a reduction in the temperature range of a liquid crystal phase. As an example of the compounds multi-substituted with fluorine atoms, those having the following structure are disclosed. ##STR2##
The dielectric anisotropy values (.DELTA..epsilon.) of the compounds (a), (b) and (c) are high in the order of (c)&gt;(b)&gt;(a). However, the compound (c) is not appreciably good in respect of a compatibility with other liquid crystalline compounds, especially, a low temperature compatibility. On the other hand, the compound (d) shows an example wherein a fluorine atom is laterally substituted on the phenyl ring to which R is attached, but this compound is not so appreciably good in respect of compatibility.